The present invention relates generally to an optical moisture sensor for detecting moisture on the surface of a transparent material, and more particularly, to a moisture sensor having wide bandwidth, high order pre-demodulation filtering for reducing the effects of unwanted noise components.
The accumulation of moisture on transparent materials, such as glass or Plexiglass, can obstruct a person's view through the material. Motor vehicles have long been equipped with motor-driven windshield wipers for clearing the moisture from the external surface of the windshield, at least within the driver's field of vision, and generally over a larger area so as to enhance one's vision through the windshield.
In most vehicles today, the windshield wiper system includes multi-position or variable speed switches which allow the driver to select a wide, if not an infinitely variable, range of speeds to suit conditions. Wiper controls are manually operated and typically include a delay feature whereby the wipers operate intermittently at selected time delay intervals.
Wiper control systems have recently been developed which include a moisture sensor mounted on one of the vehicle windows to automatically activate the wiper motor when moisture is deposited upon the surface of the window. The wiper control system including the moisture sensor are most typically mounted on the windshield, although the system may be mounted on the rear window or any other glass surface intended to be cleared of moisture. Such wiper control systems free the driver from the inconvenience of frequently adjusting the wiper speed as the driving conditions change.
Optical moisture sensors operate upon the principle that a light beam is diffused or deflected from its normal path by the presence of moisture on the exterior surface of the windshield. Typical optical moisture sensors include an emitter for emitting a light signal (hereinafter called a moisture sensing signal) which is preferably an infrared radiation signal. The moisture sensing signal is directed towards a moisture detection region on the windshield where it is reflected back by the outer surface of the windshield and into a detector. The presence of moisture on the surface of the windshield reduces the amplitude of the reflected signal. The detector converts the reflected moisture sensing signal to a pulsatile electrical signal which is then demodulated to provide a signal representing the amount of moisture in the moisture detection regions.
Bright sunlight presents a difficult working environment for an optical moisture sensor. The semiconductor emitters used in most optical moisture sensors generate a few milliwatts of optical energy. Yet, a commercially viable moisture sensor must operate day or night and should be able to sense very small droplets of moisture which only provide a very small shift in the received signal level within the moisture sensor. Sunlight impinges upon the surface of the earth with roughly 1000 watts of optical energy per square meter. The sunlight provides a broad bandwidth of intense ambient light noise which interferes with the operation of the moisture sensor.
To compensate for the ambient light, moisture sensors typically use costly optical systems designed to exclude as much ambient light as possible from the moisture sensing signal. However, it is desirable to provide a moisture sensor having an inexpensive optical system.
Some moisture sensors use electronic filters to remove the unwanted ambient light from the received moisture sensing signal as taught by the Noack (in U.S. Pat. No. 4,355,271). Noack shows a moisture sensor having a detector with an output that is connected to a first order high pass filter. The filter rejects the low frequency noise components of the signal from the detector, and provides gain to the signal before it is demodulated. However, the first order filter taught by Noack attenuates frequencies below the cutoff frequency of the filter at a relatively gradual 20 dB per decade. The gradual attenuation of the filter still allows a significant amount of ambient light noise to be present in the received moisture sensing signal when it is demodulated. The ambient light noise still creates problems with the operation of the moisture sensors in bright sunlight when shadows cause the false detection of moisture. It is desirable to reduce the false detection of moisture by shadows on bright sunny days.
Vehicle moisture sensors should be able to be used on many different windshields having different transmittances. The transmittance of the windshield glass affects the amount of light which will pass through the glass which determines the strength of the reflected emitter signal reaching the detector. For example, modern solar-control windshields, such as windshields sold under the trademark "EZ-KOOL" commercially available from Libbey-Owens-Ford Co., absorb much of the infrared energy used by many optical moisture sensors drastically reducing the strength of the reflected emitter signal. Therefore, it is desirable produce a high intensity moisture sensing signal so that the signal reaching the detector will have a sufficient intensity to provide a useable signal. Furthermore, it is desirable to produce a high intensity emitter signal to overcome the noise created by ambient light.
The intensity of the emitter signal is determined by the amount of electrical energy provided to the emitter. However, typical emitters have a limited power capacity. A larger amount of energy, typically in the form of current pulses, can be provided to the emitter for a shorter duration without surpassing the average power capacity of the emitter. The current pulses produce a pulsatile emitter signal having sufficient intensity. The pulsatile emitter signal is said to have a low duty cycle in which the signal pulses are relatively short in duration as compared to the rest of the signal interval. It is desirable to provide an inexpensive moisture sensor with a low duty cycle moisture sensing signal having filters which capture as much of the useful moisture sensing signal as possible while rejecting more of the ambient noise to improve the sensor's moisture detection capabilities.